1. Field of the Invention
The present invention generally relates to systems for decoding signals such as Class IV partial-response signals and, more particularly, to systems wherein threshold levels that are used to distinguish between binary ones and zeros when decoding partial-response signals are adjusted automatically to compensate for effects such as signal dropouts.
2. State of the Art
In the art of recording information on magnetic media, various techniques have been developed to maximize data packing densities and to improve reproduced signal response in the presence of noise. In particular, a technique known as "Class IV partial-response coding" has proven beneficial to enhance the extraction of digital information from recordings on magnetic tape. Methods for encoding information in the partial-response signal format are disclosed in Kabal and Pasupathy, "Partial-Response Signalling", IEEE Transactions on Communications, Vol. COM-23, No. 9, Sept., 1975.
In reproducing digital data that has been recorded on magnetic tape, one advantage of Class IV partial-response coding is that signal-to-noise ratios of off-tape signals are significantly improved. Also, signals encoded by Class IV partial-response coding are DC free during tape playback, i.e., the encoded off-tape signals are automatically centered on zero volts. The use of partial-response coding in magnetic recording systems is described in an article by Kobayashi and Tang, "Application of Partial-Response Channel Coding to Magnetic Recording Systems", IBM J. Res. & Devel., July, 1970.
Partial-response coding of digital signal information, however, has several drawbacks. One drawback is that the encoded partial-response signals are not binary but have three levels. Generally speaking, this problem can be overcome by full-wave rectification of the off-tape partial-response signals. A more significant problem is that changes in the amplitude of off-tape partial-response signals can adversely affect decoding accuracy. In practice, amplitude in off-tape signals occur frequently, usually as a result of dropouts. Usually, dropouts are common in magnetic recording tapes having high data packing densities.
To decode Class IV partial response signals, it is conventional to employ comparators having fixed voltages which determine threshold levels. The thresholds are usually fixed mid-way between the center level (i.e., zero volts) and the absolute value of the normal outer envelope level for the partial-response signal. (The outer envelope levels are defined with reference to ternary eye patterns for partial-response signals; the ternary eye patterns are described by Kabal, et al., supra.) With fixed decoding thresholds, however, a dropout that causes a decrease in the peak-to-peak amplitude of a partial-response signal for the duration of some number of data bits can result in a logical "1" being erroneously decoded as a logical "0" during the dropout period even in the absence of noise. With electronic noise present, decoding errors may occur even with lesser signal reductions.
U.S. Pat. No. 4,399,474 suggests that threshold voltages employed in decoding Class IV partial-response signals should be automatically adjusted in accordance with changes in amplitude of the partial-response signals. The goal of the automatic adjustment is to change threshold voltages in proportion to scale changes in the levels of the partial-response signals in the absence of noise. In particular, the patent discloses a system including first and second sample-and-hold circuits. The first sample-and-hold circuit provides an output which is updated each sampling period to represent the amplitude of the most recently sampled partial-response signal. The second sample-and-hold circuit provides an output the reproduces the output of the first sample-and-hold circuit only when the amplitude of a sampled partial-response signal equals or exceeds a predetermined fraction of the amplitude of the next-previously sampled partial-response signal. According to the patent, threshold voltages are adjusted as a function of the output signals from the second sample-and-hold circuit; that is, threshold voltages track changes in the amplitude scale of partial-response signals.
Although systems according to U.S. Pat. No. 4,399,474 are effective in minimizing decoding errors due to shallow dropouts (i.e., ones in which the partial-response signal drops only a few dB), the system is complex and expensive to implement. In part, the implementation difficulties arise because decoding requires sampling partial-response signals at high rates such as 100 MHz and, therefore, requires rapid operating cycles for the sample-and-hold circuits. While such high-speed sample-and-hold circuits can be designed, they are difficult to manufacture.